Method of protecting lead against corrosion



Patented Jan. 23, 1934 UNITED STATES..PATENT' oF ca Henri Benlt, Paris, France, assignor to .Com-

pagnie Generale DElectrlclte, Paris, France No Drawing. Appllcation'lvlay 7, 1931, Serial No. S

' 535,810, and in Germany May 7, 1930 S v 3 Claims. (Cl. 173-266) g The corrosion of lead employed in thefo'rm 'the protective'covering either in the form of of pipes or tubes and for sheathing electric pure sulphur whichdissolves well in tar or pitch cables exposed to theweather or buried in the or in the form of natural or artificial sulphide soil or embeded in cement is well known. derivatives.

5 In general, in order to prevent corrosion the For example, there may be applied to the lead 60 lead is separated from the surrounding medium directly at a' temperature a little above 100 C. by covering it with a protective coating generally,v a layer of ordinary tar mixed with pure sulphur in constituted by one or more layers of pitch or aproportion greater than 3%. Under these con- 01' tar, the coating being maintained and proditions' there will be formed instantaneously on tected mechanically by a covering of jute or the leada film-likecoating of lead sulphide and 55 canvas. It isfound in practice that such water-. the reactionwill cease of its own accord leaving proof coatings do not always remain wholly iman excess of sulphur in the tar. permeable, cracks are produced eventually in Theadvantages of the'process forming the the layer of protective coating and infiltration subject of'the invention are the following:

of' moisture in the presence of air produces corr'o- The superficial layer of lead sulphide protects sion of the lead. the lead against the action of phenols, cresols and Moreover the substance used to form the proother destructive agents contained in tar or in tective coating should not itself contain products the protective covering employed, or which may which are corrosive of lead or which become so be formed even in the presence of other chemiin the presence of other chemical agents concal agents in the soil. The lea sulphide which 75 tained in the soil; otherwise the lead will be covers the surface of the lead protects the lead destroyed eventually. It is thus necessary to even in the case where the waterproof covering employ a perfectly neutralprotective coating round the pipe or the lead sheathing is not which is much more costly in general than ordicompletely impermeable to water. The lead sulnary tars and the neutrality of which is often phide even altered by oxidation continues to prodifllcult to verify. tect the lead by being converted into sulphate The process forming the subject of the preswhich is equally insoluble or very slightly soluble ent invention avoids the above mentioned disin water. advantages. It consists in rendering the lead The excess of sulphur contained in the layer unattackable by destructive agents by creating of tar continues'to protect the lead by acting as a on its surface, chemically, a protective layer sulphidizing medium orasasulphatizing medium formed by a very stable lead salt which resists after oxidation. This is important as it might chemical changes. The lead salt thus produced happen that the first protective coating of sulwill form a very thin layer protected mechanphide would be impaired and the lead would be ically by the usual covering which might now. be exposed. Under these conditions there would of ordinary tar or other inexpensive material still exist along the lead tube the sulphidizing which wil not be chemically neutral. There will and sulphatising protective medium. .On the no longer be any liability of corrosion of the lead other hand, the constant emission of sulphuric in case of cracking of the waterproof covering acid would oppose the dissolving action which 40 even if corrosive products are present in this certain ammoniacal organic salts which are found coating. because the lead salt formed on the surin the soil (acetates, tartrates of ammonia etc.) face 01 the lead will P nt at the latter. exert onfthe sulphate of lead.

The lead salt which forms the protective Where the protective covering for the lead conshould be very stable and be easily and cheaply sists of concrete or cement another form of the 45 formed. Lead sulphide fulfils these conditions; invention consists in incorporating inthe cement it is insoluble in water and is unattackable or during the preparation of the mortarasmall proalmost unattackable by acids and mineral and portion of sodium sulphide. Itwill be underorganic salts. It is formed easily from pure sulstood, from what has been stated above, that the phur or from natural or artificial sulphide delead will thus be protected against attack of the rivatives in contact with lead, the reaction being most highly basic cements. This form of the very rapid with the aid of heat. invention is susceptible of very important appli- In accordance with the invention the proteccations because it permits of avoiding the cortive layer is formed perferably by lead sulphide rosion of the lead in the very frequent instances obtained simply by introducing an excess of sulwhere lead pipes pass through walls or masonry 55 phur into the substance intended to serve as or when electric cables covered with lead sheathno in: are laid on shelves 0! cement in galleries or in sewers.

The process forming the subject of the present invention permits also of protecting the lead from electrolytic corrosion or at least of reducing greathr the destructive effects of electrolysis by retarding the speed of the reaction in the ratio of about 10:1 under the same conditions. In fact the oxygen liberated by electrolysis transi'orms the sulphide of lead into the sulphate which continues to protect the lead. In the presence of an excess of sulphur in the tar sulphuric acid will be formed which will assist to maintain the protective medium in contact with the lead in the sulphidizing and sulphatizing state.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:

1. In a method of preventing corrosion of the lead sheathing of electric cables wherein the lead sheathing is protected against mechanical deterioration by a layer of a suitable covering material not chemically neutral and not adapted to protect the lead against corrosion, incorporating sulphur to the said protective materials in such quantity that on contact of the lead with these materials a him of lead sulphide will be immediately formed thereonand there will remain in the said protective materials an excess of sulphurating material constantly active to renew any worn portion of the said lead sulphide him, and so prevent the destructive eflects of electrolytic phenomena on the lead.

2. In a method 0! preventing corrosion of the lead sheathing of electric cables wherein the lead sheathing is protected against mechanical deterioration by a layer of a suitable covering material not chemically neutral and not adapted to protect the lead against corrosion, incorporating to the said protective materials quantities of sulphur derivatives having a sulphurating action on lead such that on contact of the lead with these materials there will be immediately formed on the lead a film of lead sulphide and there will still remain in the said protective materials an excess of sulphurating material active to constantly renew any worn portion of the said lead sulphide film, and so prevent the destructive eilects of electrolytic phenomena on the lead.

3. In a method of preventing corrosion oi the lead sheathing of electric cables wherein the lead sheathing is protected against mechanical deterioration by a layer 01 a suitable covering material not chemically neutral and not adapted to protect the lead against corrosion, incorporating to the said protective materials a quantity oi! sodium sulphide such that on contact of the lead with these materials a film oi lead sulphide will be immediately formed on the lead sheathing and there will still remain in the said protective materials an excess of sulphurating material constantly active to renew any worn portion or the said lead sulphide film, and so prevent the destructive effects of electrolytic phenomena on the lead.

HENRI BENIT. 

